1. Field of the Invention
The present invention relates to a copolymer of propylene and butene. More specifically, the present invention relates to a copolymer of propylene and butene-1 which has a specific structure.
2. Description of the Prior Art
Although the existence of syndiotactic polypropylenes has been known from old days, those produced by the conventional process in which propylene is polymerized at a low temperature in the presence of a catalyst comprising a vanadium compound, an ether and an organoaluminum, are of low syndiotacticity and hence can hardly be regarded as syndiotactic polypropylenes. Copolymers of other olefins with propylene are much less regarded as crystalline polypropylenes. On the other hand, a polypropylene of good tacticity, say, a syndioactic pentad fraction of more than 0.8, has been discovered for the first time by J.A. Ewen et al. by the use of a catalyst comprising a transition metal compound having an asymmetric ligand and an aluminoxane (J. Am. Chem. Soc., 1988, 110, 6255-6256).
The above-mentioned method of J.A. Ewen et al. exhibits a high catalytic activity per unit amount of the transition metal and can provide a polymer having a high tacticity and relatively properly balanced physical properties. However, molded articles of the polymer are poor in transparency and unsatisfactory in impact resistance at low temperatures.